THIS INVENTION relates to the treatment of fluorocarbon feedstocks. It relates in particular to a method of treating a fluorocarbon feedstock.
According to the invention, there is provided a method of treating a fluorocarbon feedstock, which method includes
heating, by means of radio frequency induction, a heating zone to a high temperature;
allowing a fluorocarbon feedstock, comprising at least one fluorocarbon compound, to heat up in the heating zone so that the fluorocarbon compound dissociates into at least one fluorocarbon precursor or reactive species; and
cooling the fluorocarbon precursor or reactive species, thereby forming, from the fluorocarbon precursor or reactive species, at least one more desired fluorocarbon compound.
The heating zone may thus be provided by a reactor. The reactor may comprise an elongate cylindrical reactor shell providing a reaction chamber which contains the heating zone, and a feedstock holder in the heating zone of the reaction chamber. The reactor shell typically is of quartz, and may have its ends sealed off and water cooled.
The radio frequency induction heating-may be provided by a radio frequency induction heating oven having an induction coil within which the heating zone of the reactor is located. In other words, the induction heating coil is located around that part of the reactor shell containing the heating zone.
In one embodiment of the invention, the reactor shell may extend vertically and be stationary. It is believed that this configuration will be particularly suited to treating feedstock in the form of unfilled not directly usable material as hereinafter described.
However, in another embodiment of the invention, the reactor shell may be tilted at an angle to the vertical, eg between about 5xc2x0 and about 60xc2x0 to vertical, and it may rotate or vibrate. The reactor may then be provided with a graphite crucible having transverse baffles to regulate the residence time of the feedstock in the reactor. It is believed that this configuration will be particularly suited to treating feedstock in the form of filled material, which is not directly usable as hereinafter described; as the filled material passes downwardly down the reactor, it is depolymerized and evaporates, thus passing upwardly out of the reactor, while filler material passes downwardly out of the bottom of the reactor. Instead, an upright reactor can be used to treat filled material; however, the reactor will then be provided, at its lower end, with a removable plug to drain filler material.
The feedstock may, at least in principle, be in gaseous, liquid or solid particulate form, or in the form of mixtures of two or more of these. When the feedstock is in liquid form, it may be a more-or-less pure feedstock comprising a single fluorocarbon compound, such as C6F14; however, it is envisaged that the feedstock will then normally be a not directly usable fluorocarbon product comprising two or more of a range of fluorocarbon compounds such as C5F12, C6F14 C7F16, C8F18, C4F8, C8F 6, (C3F7)3N, C6C13H, C6F12H2, or the like. Normally, one compound will be present in such a product as a dominant component, ie constitute the major proportion of such a product. The feedstock may then be fed into the reactor from the bottom.
When the feedstock is in solid particulate form, it may, in particular, be a filled or an unfilled not directly usable material such as polytetrafluoroethylene (xe2x80x98PTFExe2x80x99), tetrafluoroethylene hexafluoropropylene vinylidenefluoride (xe2x80x98THVxe2x80x99), fluorinated ethylene-propylene copolymer (xe2x80x98FEPxe2x80x99), perfluoroalkoxy copolymer (xe2x80x98PFAxe2x80x99), or the like. By xe2x80x98filledxe2x80x99 is meant that the fluorocarbon feedstock may contain elements or substances such as silica, copper, carbon, etc which were originally added to fluorocarbon material to impart specific properties thereto. Once such material has been used and has thus become, mechanically, not directly usable material, but suitable for use as the feedstock in the method of the invention, it will still contain these filling elements. In the method of the invention, these materials are depolymerized, and the more desirable fluorocarbon compound formed therefrom. The feedstock may then be fed into the reactor from the top or from the bottom.
If desired or necessary, the solid particulate feedstock may be pretreated to remove surface contaminants such as oil and dirt, eg by means of solvent extraction.
Typical products which may be obtained are tetrafluoromethane (CF4), tetrafluoroethylene (C2F4), hexafluoroethylene (C2F6), hexafluoropropylene (C3F6), fluorobutylene (C4F6), cyclic octafluorobutylene (c-C4F8), decafluorobutylene (C4F10), octafluoropropylene (C3F8) and other CxFy chains where x and y are integers.
The reactor may operate on a batch, on a semi-continuous, or on a continuous basis. The method will thus include feeding the feedstock into the reactor zone on a batch, on a semi-continuous, or on a continuous basis. By xe2x80x98batchxe2x80x99 is meant that a predetermined quantity of the fluorocarbon is loaded into the reactor and allowed to react to completion with the hot plasma gas. By xe2x80x98semi-continuousxe2x80x99 is meant that a hopper is filled with feedstock, with this feedstock then being fed into the reactor at a continuous, normally constant, feed rate until the hopper is empty, whereafter the hopper may be refilled. By xe2x80x98continuousxe2x80x99 is meant that the feedstock is fed continuously into the reactor, normally at a more-or-less constant feed rate.
While the feedstock may, in principle, be introduced into the cavity or the first zone of the reaction chamber in any desired manner, gravity feed may, in particular, be employed since relatively large feedstock particles can thereby readily be used, eg particles in the size range 1 to 10 mm, preferably 3 to 5 mm. Thus, the feedstock may be fed vertically into the chamber under gravity, immediately above the heating zone.
The cooling of the fluorocarbon species or precursor may be effected in a second zone of the reaction chamber located above the heating or first zone thereof. The cooling may be effected by means of a quench probe, which may be a self-cleaning probe. The self-cleaning quench probe may comprise an outer cylindrical component mounted to the reactor, providing a central passageway and adapted to cool the hot gas passing through the passageway; a plurality of circumferentially spaced elongate teeth or scrapers protruding inwardly from the outer component into the passageway; an inner cylindrical component located with clearance inside the outer component, with the inner component also adapted to cool the hot gas passing along the peripheral gap between the components; a plurality of circumferentially spaced elongate teeth or scrapers protruding outwardly from the inner component into the passageway, with these teeth or scrapers being staggered with respect to the teeth or scrapers on the outer component; and drive means for driving the one cylindrical component to oscillate relative to the other cylindrical component. The drive means may, for example, comprise a spring loaded piston driven arm.
Instead, however, any other suitable quenching means can be used such as rapid expansion of the product gas, gas quenching by means of another gas which is cold, or the like.
The reaction chamber may be operated under pressures ranging from near vacuum to elevated pressures, depending on the more desired fluorocarbon compound required as product and other process variables. Evacuation may be effected through the quench probe.
Normally a spread of fluorocarbon compounds will form as products. The method may then include separating the various products from one another.
According to a second aspect of the invention, there is provided a quench probe which comprises
an outer cylindrical component providing a central passageway and adapted to cool a hot gas passing through the passageway;
a plurality of circumferentially spaced elongate teeth or scrapers protruding inwardly from the outer component into the passageway;
an inner cylindrical component located with clearance inside the outer component, with the inner component adapted to cool the hot gas passing along the peripheral gap between the components;
a plurality of circumferentially spaced elongate teeth or scrapers protruding outwardly from the inner component into the passageway, with these teeth or scrapers being staggered with respect to the teeth or scrapers on the outer component; and
drive means for driving the one component to oscillate relative to the other component.
The inner component may be located centrally or concentrically within the outer component. The same number of teeth or scrapers may be provided on the inner and outer components. The teeth or scrapers may be spaced equidistantly apart on their components. The teeth or scrapers may extend parallel to one another.
The components may be hollow and/or may be provided with passages to permit a cooling fluid, such as water, to pass through them in order to cool or quench the hot gas.
The drive means may, as also hereinbefore described, comprise a spring loaded piston driven arm attached to one of the cylindrical components.
Due to the oscillation of the one component relative to the other, cleaning of solidified or sublimated material from the surfaces thereof, on passage of the gas through the annular gap between the components, is achieved.
The quench probe is particularly suited for use in a reactor as hereinbefore described; however, it is not limited only to such use. Normally, the outer component will be fixed to the reactor, with the inner component oscillating relative to the outer component.
The invention will now be described in more detail with reference to the accompanying simplified flow diagrams.